Optical communication typically involves an optical signal such as a beam of light in the ultraviolet, visible, or infrared wavelengths that has a characteristic that is varied or modulated. This modulated characteristic may be the intensity (amplitude), the phase, the frequency, the polarization, the location or duration of a pulse of light, and so forth. The pattern of modulation represents the information conveyed on the optical communication system. At one end of the optical communication system may he a source of light and a means for modulating the tight output from the source. At another end of the optical communication system may be an optical receiver and a means for demodulating the received optical signal.
Information in a communication system is conveyed by first representing the information in some unambiguous digital form, where unambiguous means that the conversion from the information to the digital form and from the digital form back to the information is one-to-one. For example, one such mapping is the ASCII encoding of text. The digital form is then used to select members from a set of (usually orthonormal) basis vectors (or basis functions) to represent the digital data. This process falls under the general topic of geometrical representation of signals (see Section 7.1, page 548 of Modern Digital and Analog Communication Systems, B. P. Lathi, Holt, Reinhart and Winston, 1983 and/or Section 3.2, page 157 of Digital Communications, Second Edition by John G. Proakis, McGraw-Hill, 1989). The complete set of basis vectors used is typically called the signal constellation.
The M in M-ary comes from the use of M such basis vectors in the signal space. When M=2, then there are 2 such basis vectors, one is using Binary signaling, and is sending one bit per symbol. When M=4, then there are 4 such basis vectors, one is using Quaternary signaling, and is sending 2 bits per symbol. When M=8, one is sending 3 bits per symbol, and so on. In general the number of bits per symbol is given by computing the logarithm base 2 of M (Lathi, op. cit. page 177). The usual use of the term M-ary refers to signal constellations with more than 2 basis vectors: that is, M>2.
Bandwidth efficient modulation (BEM) techniques have been developed in response to the need for ever-higher communication link data rates in the presence of fixed bandwidth frequency authorizations for the transmission of the data. The essential relationship between data rate and bandwidth is encapsulated in the term Bits per Hertz. Each modulation type has a definable value of Bits per Hertz. For example, Binary Phase Shift Keying (BPSK) has a Bits per Hertz value of 0.5, while Quaternary Phase Shift Keying (QPSK) has a Bits per Hertz value of 1.0. Bandwidth efficient modulation techniques are able to achieve Bits per Hertz values of more than 4. The penalty for using BEM techniques is twofold. The modulator and demodulator used in generating the transmitted signal and in recovering the data from the received signal have increased technical complexity. And BEM techniques require ever higher received signal power in order to provide the same bit error rate as the value of Bits per Hertz increases. This higher received signal power must be made up through a combination of higher transmitter power and higher antenna gains (requiring larger transmit and receive antenna sizes). These two penalties increase the expense of building and maintaining the communication link.
Conventional methods that implement BEM communications systems generate the signal constellation at intermediate radio frequencies (IF) using vector modulators, usually driven by the output of a pair of digital-to-analog (D/A) converters, which are, in turn, driven by the output of programmable read-only memories (PROMs). The input to the PROMs is the bitwise representation of the BEM symbol to be converted. This technique may be used to generate any of several popular signal formats, such as PSK (4-, 8-, 16, . . . ), QAM (4-, 8-, 16-, 32-, 64-, 128, 256, . . . ), 12, 4 SQAM, and others including BPSK, QPSK, and DPSK.
An SQAM method is disclosed in U.S. Pat. No. 4,644,565 granted to Dr. J. S. Seo and Dr. K. Feher. U.S. Pat. No. 5,313,494 granted to Park et al. improves aspects of the Seo et al. modulation method, such as by reducing a probability of error. The contents of each of these patents is incorporated herein by reference.
The enormous bandwidths of optical (or photonic) communications systems, and a desire for low cost systems, have resulted in a present commercial use of relatively simple signaling equipment and simple modulation types. Most ground-based and space-based commercial and military systems use amplitude modulation, either by directly modulating the intensity of the laser light of a laser diode, or by using Mach-Zehnder modulation of a master laser, to modulate at the transmission end of the optical communication system, as is well known. At the receiver end of the optical communication system, the conventional method then uses direct photodiode detection of the intensity variation to recover the original signal modulation waveform. This approach, in general, is used in all the commercial multi-gigabit per second communication links, including those employing wavelength division multiplexing (WDM) to combine multiple data links on a single optical fiber. This conventional technique is also used for vestigial-sideband modulated (VSM) multi-channel cable TV over optical fiber networks. The communication method used in these systems is called intensity modulated, direct detection (IMDD). This communication signaling method is known as a “non-coherent modulation” technique.
On the other hand, it is well-known in the communications arts that coherent modulation techniques offer a 3 dB to 6 dB reduction in the amount of carrier power required to achieve the same bit error rate (BER) as compared to non-coherent modulation techniques. Further, when one is bandwidth constrained, coherent modulation techniques can pass four or five times the data in the same bandwidth at the same BER than one can pass using non-coherent techniques. A problem with conventional coherent modulation techniques, however, is that they all require the use of a phase locked loop to implement the demodulation.
There are some quasi-coherent techniques that use the signal itself as a phase reference, such as by encoding the data so that the phase change between successive signals is the important parameter. This quasi-coherent technique allows the current bit to be decoded by using a time-delayed version of the previous bit, where both bits are then fed to a phase comparator. Differential phase shift keying (D-PSK) is an example of this quasi-coherent modulation technique. A simple, high-performance implementation of a D-PSK modulator and demodulator that can be used at optical frequencies has been developed by MIT/Lincoln Labs (E. A. Swanson, J. C. Livas, and R. S. Bondurant. “High Sensitivity Optically Preamplified Direct Detection DPSK Receiver with Active Delay-Line Stabilization.” IEEE Photonics Technology Letters, Vol. 6, No. 2. pp 263–265, Feb. 1994).
Turning again to coherent modulation, development of the phase locked loop required for demodulation of coherent signaling formats at radio frequencies has become a highly specialized technical specialty within the communications field. Phase locked loops at radio frequencies are generally based on a voltage-controlled oscillator (VCO), a radio frequency generator whose output frequency depends on the value of an input voltage. A central part of the conventional VCO is a varactor diode, which is a voltage-variable reactor-capacitor that is constructed using a back-biased diode. The varactor is bridged across the oscillator's tank circuit, which may be an inductor-capacitor resonant type circuit, a mechanical crystal type circuit, or a multi-vibrator resistor-capacitor time constant type circuit. The varactor can be modulated at a high rate, allowing it to closely follow the frequency and phase of the signal that is being locked into the phase locked loop. Unfortunately, there is no similar device that operates at optical frequencies to make the optical output frequency a function of an applied voltage. Low speed devices have been built that use the piezo-electric effect to physically adjust the length of a laser cavity. However, these piezo devices have not demonstrated the tracking speed required to allow the optical phase locked loop to lock to the phase of an input signal.
Traditionally, when using a coherent modulation scheme at optical frequencies and when optimizing performance of the communication system for highest sensitivity, complex (where the term complex is used in the mathematical sense of the real and imaginary representation of orthogonal baseband signals—the in-phase and quadrature components) amplitude and phase demodulation is performed at optical frequencies using homodyne detection. This requires the use of an optical phase locked loop. Since an optical phase locked loop demodulator of this type is expensive and difficult to manufacture, coherent modulation is not practical for the commercial market, but has potential applicability for military applications where performance is often more important than cost.
Coherent modulation techniques have been developed in the field of laser communications. Binary amplitude shift keyed, binary phase shift keyed, and quadrature phase shift keyed signals have conventionally been generated using Mach-Zehnder interferometers as modulators. The generation of phase shift keyed signals has conventionally taken advantage of the fact that the early Mach-Zehnder modulators had a simultaneous amplitude and phase modulation characteristic. The simultaneous amplitude and phase modulation was a result of an inability to make the two legs of the Mach-Zehnder modulators exactly equal in terms of the number of wavelengths. This inability was due to limitations in the state of the art of thin film lithography. However, by using a single Mach-Zehnder modulator, four equal amplitude signals with phases at 0, 90, 180, and 270 degrees could be produced. By using signals spaced at 180 degrees, binary phase shift keyed signals could be generated. By using signals spaced at 90 degrees, the four signal states needed to represent quadrature phase shift keyed signals were available.
Improvements in lithography and in precision of manufacturing have resulted in the production of a zero phase Mach-Zehnder modulator, where the length of the legs of the interferometer are matched. This development was driven by technical requirements flowing out of competition in the commercial marketplace. In order to implement dense WDM fiber optic communication systems even when using low dispersion or dispersion compensated fibers, it is necessary to minimize the bandwidth of each of the signals. The presence of simultaneous amplitude and phase modulation on intensity-modulated optical carriers, when transmitting using existing optical fibers over significant distances, may cause inter-symbol interference in the communication channel, which increases the bit error rates to values that are unacceptable for commercial quality-of-service (QOS) standards. The simultaneous amplitude and phase modulation may also cause adjacent channel interference that also produces elevated values of bit error rates. These problems of the spreading of the signal symbols with distance and the spreading of energy into adjacent channels are reduced if there is no phase modulation accompanying the amplitude modulation.
In ground-based optical communication systems, it is current practice to recover data to the electronic signaling level after each link span. The recovered data is then passed to an electronic cross-connect if the link-to-link node is a relay or repeater node. The output of the electronic cross-connect is then passed into the modulator for the next link. If the link-to-link node is a routing node, then the recovered data is passed to the input of a switch. Depending on the destination of the packets, the output of the switch can go to one of N possible output ports.
In optical communication systems between satellites in space, known as inter-satellite links (ISL), conventional signaling methods require the demodulation and remodulation of the data aboard a satellite, which then also requires a high data rate digital switch or space-qualified digital router. Unfortunately, these data handling/data routing components are excessively heavy in weight and consume too much power.
Another issue with ISL systems relates to transitions between RF and optical communications systems. In order to help reduce the size of a satellite link, conventional RF communication relay satellites may use a transponder/bent-pipe architecture which only requires an IF switch matrix to route data to other crosslinks to the downlink. For example, U.S. Pat. No. 5,825,325 granted to O'Donovan et al., incorporated herein by reference, discloses combining an intersatellite link with on-board subchannel switching capability, so that a bent-pipe intersatellite linked system maximizes the bandwidth efficiency of the intersatellite link. However, an optical ISL architecture which is compatible with the RF bent-pipe architecture is needed in order to assist in a transition for military space communication systems from RF to optical links. In conventional systems, there is no method for a heterodyne-up conversion from an IF frequency to a photonic frequency.
It is against this background and with a desire to improve on the prior art that the present invention has been developed.